Conventional vaccine approaches for AIDS based on delivery of HIV-1 envelope (Env) proteins or peptides derived from Env sequences have failed to generate broadly neutralizing antibodies (bNABs) to the virus, which mutates rapidly to escape from the immune response. Recently, it has been shown that some individuals develop antibodies targeting conserved epitopes on the virus that have broad, cross-clade neutralizing activity. A series of human monoclonal antibodies have been generated from these individuals that target three conserved regions on the Env gp120 glycoprotein, including the CD4 binding site (CD4bs), the V2/V3 loops, and a new glycan-dependent region. Despite the availability of bNAbs, construction of complementary immunogens has been problematic because (1) the neutralizing epitopes are conformational and it has been challenging to construct immunogens that faithfully replicate the natural epitope and are structurally constrained in the desired conformation;and (2) non- neutralizing epitopes in vaccine constructs are immunodominant and direct the immune response away from neutralizing epitopes. Rather than concentrating on peptide mimetics, which have largely failed to induce neutralizing antibodies, we investigated whether small molecule haptens could serve as the basis for vaccine development. Previous results (Caulfield, J. Biol. Chem. 2010) documented that antibody-binding small molecules could be selected by probing a large chemical library with D5, a human monoclonal antibody (mAb) that binds to a pre-hairpin fusion intermediate on gp41 of HIV-1. We further showed that molecules discovered in the HTS could be rendered immunogenic by conjugation with a carrier protein thereby forming the basis for development of AIDS vaccine candidates. The D5 mAb used in the published studies is not very potent and lacks sufficient breadth. Therefore, we propose to utilize the new more potent bNAbs to test the hypothesis that individual bNAbs can bind specifically to diverse chemical entities that can be selected in a high throughput screen of a large chemical collection. The objective of this proposal is to develop and implement screening assays to utilize existing human mAbs demonstrated to have broad neutralizing activity (bNAbs) to screen a large and diverse small molecule library (in collaboration with the Scripps MLPCN Molecular Screening Center) to identify and characterize haptens that bind specifically to the antibody combining site of the bNAb used for the screening assay. Small molecule mimetics of the HIV-1 envelope protein identified in this screening program could be useful in development of an AIDS vaccine. PUBLIC HEALTH RELEVANCE: There is a clear, unmet medical need to discover and develop novel approaches to an AIDS vaccine. This proposal will exploit the antigen-binding property of newly discovered broad and potent neutralizing antibodies to select complementary small molecule leads that can be used as building blocks for the development of a vaccine to prevent infection with the HIV-1 virus.